The invention is related to the technology of the deposition of composite surface systems possessing high resistance to wear, erosion and chemicals. More specifically, the invention is related to the technology of the deposition of coatings containing tungsten carbides and mixtures of them with each other and with tungsten or free carbon.
Superhard erosion and corrosion resistant coatings, including those containing tungsten carbides, are widely used in manufacturing various articles of tools in present-day mechanical engineering. Such coatings possess high resistance to erosion, chemicals and wear, and thus substantially increase the life of mechanical engineering products and of tools operated under demanding conditions.
Patent GB 2 179 678 describes a surface composite system with high resistance to wear and erosion consisting of a mixture of tungsten (for plasticity) and tungsten carbide W2C (for hardness). These hard coatings made from a fine-grain mixture of tungsten carbide with metallic tungsten were obtained by means of physical vapour deposition (PVD) by spraying tungsten and carbon from separate sources. The tungsten and carbon are condensed on different-type substrates to form the said alloys of tungsten with tungsten carbide.
However, the rate of synthesis of tungsten carbides is very low, and internal stresses in the coatings increase sharply as the tungsten-carbon layer grows, resulting in delamination of the coatings. For this reason, it is impossible to produce sufficiently thick coatings by the PVD method. Furthermore, the physical vapour deposition method is intrinsically inapplicable for deposition of coatings on items of complex shape due to the impossibility of depositing the coatings on the parts of the item shadowed relative to the incident beam.
The chemical vapour deposition process (CVD) eliminates these disadvantages. The CVD process can be used to deposit wear and erosion resistant coatings on substrates and on items of complex shape.
In a typical CVD process for the deposition of composite coatings, the substrate is heated in the reaction chamber, and the previously mixed gas reagents are then introduced into this chamber. By varying the composition of the reaction mixture and of the parameters of the process (temperature of the substrate, composition of the reaction mixture, flow rate, total pressure in the reaction mixture, temperature of the gases supplied, etc.), it is possible to obtain a variety of coatings.
Among the CVD methods of tungsten carbide coating deposition, only the fluoride method makes it possible to form tungsten carbides of high quality at a low temperature. For this purpose, one may use thermal decomposition of a mixture of tungsten hexafluoride, hydrogen and carbon-containing gas in the CVD process.
Various reagents were used as carbon-containing gases, e.g. dimethylether, amines, propylene, etc., with the aid of which one may synthesise tungsten carbide of one or two compositions.
For example, the thermal decomposition of dimethylether (DME) (EP 0 328 084 B1) results in the formation of the mixture W+W3C; W+W2C+W3C; W+W2C in the form of bilaminar coatings. The internal tungsten layer of the coating is obtained from the as mixture WF6 (0.3 l/min), H2 (3 l/min), Ar (4.0 l/min) at 460xc2x0 C. The external layer containing a mixture of tungsten with W3C is obtained from a mixture of WF6 (0.3 l/min), H2 (3 l/min) and DME (0.4 l/min) at 460xc2x0 C. at a total pressure of 40 torr. The external coating W+W2C is obtained from a mixture of WF6 (0.3 l/min) and DME (0.55 l/min) at 460xc2x0 C. at a total pressure of 40 torr. The external coating W+W2C is obtained from a mixture of WF6 (0.3 l/min), Ar (4.5 l/min) and DME (0.85 l/min) at 460xc2x0 C. and a total pressure of 40 torr.
Patent JP 9113527 A 19910204 describes how tungsten carbide WC is obtained from a gaseous mixture of WF6, H2 and amines with an atomic ratio of C to N equal to 1:20 and H to W equal to 1:15 at 400-900xc2x0 C. The patent cites the production of WC from the mixture WF6:trimethylamine:H2=1:2:3 (the atomic ratios are C/W=6.0, H/W=6.0). The flow rate is 120 cm3/min at 800xc2x0 C. and the total pressure is equal to atmospheric. A 70 xcexcm layer forms in 30 minutes.
Patent JP 8857301 A 19880310 describes how a W3C coating on an aluminium substrate is obtained from a gaseous mixture of WF6, H2 and aromatic hydrocarbon with atomic ratios C/W equal to 2-10 and H/C exceeding 3 at temperature 250-500xc2x0 C.
Patent JP 84280063 A 19841228 describes how a W2C coating on a graphite substrate is obtained from a gaseous mixture of WF6, C3H6 and H2 with inert gas. The preferred regime:mixture WF6:H2=1:3-1:15 with an admixture of C3H6 in the reaction mixture with molar ratio 0.01-0.3; the temperature of the substrate is 350-600xc2x0 C.
Patent JP 84204563 A 19840929 describes how a W2C coating is obtained from a gaseous mixture of WF6, H2 (molar ratio WF6:H2=1:3-1.15) and cyclopropane with molar ratio in the mixture 0.01-0.3 at a substrate temperature of 350-600xc2x0 C. The example cited is the manufacturing of a W2C coating on a copper substrate from the mixture WF6: 40, H2: 320, Ar: 40, C3H8: 10 cm3/min at 500xc2x0 C. with a growth rate of 3.3 xcexcm/min.
EP A 0 305 917 describes how super-hard fine-grain non-columnar laminar tungsten-carbon alloys are obtained by chemical vapour deposition. The described alloys contain carbide phases consisting of W2C or W3C or mixtures of them with each other. It is demonstrated that these tungsten carbon alloys, when deposited on certain types of substrate, have a net of very fine micro-cracks all over the deposit. Coatings made from these alloys have inadequate resistance to wear and erosion.
EP 0 411 646 A1 describes a multilayer coating containing alternating layers of tungsten and a mixture of tungsten with tungsten carbides in the form of W2C, W3C or a mixture of them. It is demonstrated that such a coating increases the resistance of the material to wear and erosion. It is known, however, that the maximum composition effect is observed for layers with a distinct boundary between them. This is obviously not the case for the conjunction of layers of tungsten and the mixture of tungsten with tungsten carbide which is characteristic of this patent.
It follows from the patents cited above that different reagents and different technologies are used for the production of different types of tungsten carbides. In this connection, the main aim of this invention is to develop a universal technology making it possible to obtain all the known carbides, mixtures of them and also new carbides.
Furthermore, the problem of increasing the hardness of tungsten carbide coatings remains very important, because such key parameters as strength and wear resistance are related specifically to hardness.
A solution to these and other problems is provided by this invention, due to the development of a new method for the production of tungsten carbides and mixtures of them. The main distinguishing feature of the method is the preliminary thermal activation of the hydrocarbons used in the CVD process. The synthesis of a tungsten carbide layer of a certain composition depends on an activation temperature that varies from 500 to 850xc2x0 C., on a total pressure in the reactor that varies from 2 to 150 kPa, and on the partial pressure of the hydrocarbon reagent.
Preliminary activation of the hydrocarbons results in the formation of the necessary concentration of hydrocarbon radicals and their associates with fluorine in the gaseous phase over a wide range. The proposed method makes it possible to alloy the carbides and/or mixtures of them with fluorine and fluoride-carbon compositions. Fluorine, as the most active chemical element, strengthens the interatomic bonds when it penetrates into the carbide lattice. It is the strengthening of the interatomic bonds in the carbide which produces the increase in hardness. This process is similar to the formation of oxycarbide phases instead of purely carbide structures. On the other hand, fluorine stabilises the structure of the low-temperature phases (tungsten subcarbides) due to the high energy of the fluorine-carbon bond.
Along with fluorine as an element, fluorine-carbon compounds with carbon content up to 15 wt % and fluorine content up to 0.5 wt % can be introduced into the composition of the tungsten carbide. These admixtures have two roles: firstly, they increase the hardness of the tungsten carbides; and secondly, they stabilise the structure of the tungsten subcarbides. Thus, the introduction of fluorine and fluorine-carbon admixtures makes it possible to obtain such tungsten carbides as monocarbide WC, semicarbide W2C and subcarbides W3C and W12C.
The application of the new tungsten carbides makes it possible to manufacture a bilaminar coating, the internal layer of which (deposited on the substratexe2x80x94a construction material or items made of it) is composed of tungsten. The external layer contains tungsten carbide alloyed with fluorine and possibly with fluorine-carbon compositions, or mixtures of such carbides with each other and also with tungsten and free carbon.
The construction material with the deposited composition coating has an internal tungsten layer of thickness 0.5-300 xcexcm. The thickness of the external layer is 0.5-300 xcexcm. The ratio of thicknesses of the internal and external layers ranges from 1:1 to 1:600.
In accordance with this invention, tungsten carbides are deposited in the chemical reactor on the substrate from a gaseous phase consisting of tungsten hexafluoride, hydrogen, a carbon-containing gas (e.g. propane), and, optionally, an inert gas (e.g. argon). The carbon-containing gas is thermally activated before being introduced into the reactor by heating it to 500-850xc2x0 C. The pressure in the reactor ranges from 2 to 150 kPa. The substrate is heated to temperature 400-900xc2x0 C. The ratio of carbon-containing gas to hydrogen ranges from 0.2 to 1.7, and the ratio of tungsten hexafluoride to hydrogen ranges from 0.02 to 0.12.
Within the stated limits, the parameters of the process are determined depending on which carbide or mixture of carbide with each other or with tungsten or with carbon is required to be produced. Thus, to produce tungsten monocarbide WC, the preliminary thermal activation of the carbon-containing gas is conducted at a temperature of 750-850xc2x0 C. The ratio of propane to hydrogen is set in the interval 1.00-1.50, and the ratio of tungsten to hydrogen in the interval 0.08-0.10.
The corresponding parameters for the production of single-phase tungsten semicarbide W2C are as follows: 600-750xc2x0 C., 0.75-0.90 and 0.06-0.08. The parameters for the production of tungsten subcarbide W3C are: 560-720xc2x0 C., 0.60-0.65 and 0.050-0.055.
A previously unknown tungsten subcarbide, W12C, with hardness 3500 kG/mm2, greater than that of any of the known carbides, was obtained by the method proposed in this invention. For the production of this subcarbide, propane was thermally activated at temperature 500-700xc2x0 C. The ratio of propane to hydrogen was within the interval 0.35-0.40 and that of tungsten hexafluoride to hydrogen was 0.040-0.045.
This process makes it possible to obtain mixtures of tungsten carbides and mixtures of the carbides with free tungsten and carbon. The values of the parameters for these cases are shown in Table 1.
As mentioned above, control of the content of active hydrocarbon radicals within wide limits is provided by means of the preliminary thermal activation of the initial carbon-containing reagent. This makes it possible to form carbide phases and mixtures of them with free carbon content of up to 15 wt %. The thermal activation of the carbon-containing reagent takes place in a hydrofluoric atmosphere, which provides additional formation of fluorine-carbon radicals. Radicals of both types take part in alloying the carbide phases and mixtures of them with fluorine and carbon, producing an increase in their hardness and enhanced tribotechnical properties.
Internal stresses increase slowly as the coatings of single-phase tungsten carbides grow; thus, high wear resistance is observed even with quite thick coatings (up to 300 xcexcm). Their chemical resistance and high hardness are due to the strong interatomic bonds in the carbide lattice and the absence of free tungsten.
In order to bring about a microplastic effect in the coatings, one can use mixtures of carbides with each other and mixtures of them with tungsten and free carbon, in this case losing some chemical and electrochemical stability. Note that coatings of tungsten carbide with free carbon have a reduced friction coefficient in addition to the said microplastic effect. This is very important where mixtures of carbides with free carbon are used as wear-resistant tribotechnical coatings in friction assemblies.
By using the proposed invention and also the described new method of coating deposition, one can also obtain multilayer coatings with alternating layers of tungsten and layers containing tungsten carbides alloyed with fluorine and possibly with fluorocarbon compositions, including mixtures of these carbides with each other and with tungsten or carbon. The ratio of thicknesses of the alternating layers ranges from 1:1 to 1:5.
The construction material itself, with a bilaminar or multilayer coating deposited in accordance with the proposed method, is also an object of this invention.